1. Field of the Invention
This invention relates to a liquid crystal display apparatus and, more particularly, it relates to a liquid crystal display apparatus comprising a micro-lens array.
2. Related Background Art
Known micro-lens arrays include those that can be formed on a transparent substrate covered by a light-shielding film having a given pattern as disclosed in Japanese Patent Application Laid-Open No. 7-261164. This patent document describes a method of producing micro-lenses by utilizing the surface tension of molten resin. However, lenses produced by such a method gives rise to aberration to a considerable extent in a peripheral area of the lens. This aberration problem that occurs in a peripheral area of micro-lens is normally bypassed by using a light-shielding underlayer. However, such a technique is accompanied by problems, which will be discussed below.
FIG. 13 of the-accompanying drawings is a schematic cross sectional view of a structure comprising known micro-lenses. Such a structure is typically prepared by forming a light-shielding layer 702 on a transparent substrate 701, applying a photosensitive resin material onto the substrate and thereafter exposing it to light and developing the latent image to leave the photosensitive material as dots only in areas corresponding to the pixel array to be used with it. The photosensitive resin left on the substrate as dots is then molten and transformed into micro-lenses by utilizing the surface tension of the resin material to produce a structure 703 as shown in FIG. 13. While rays of light such as those denoted by 704, 705, 706 in FIG. 13 that enter a micro-lens at and near the center thereof may be converged to the focal point 707 of the lens, those entering the micro-lens at a peripheral area thereof such as ray of light 708 is not accurately brought to the focal point 707 but follows the route indicated by 709 because of the aberration of the micro-lens is particularly remarkable in the peripheral area. As a result, the converged rays of light show a light spot having a relatively large diameter. While, theoretically, the ray of light 708 is supposed to be blocked by the light-shielding layer 702 after entering the corresponding micro-lens 703, it actually passes through the corresponding aperture of the light-shielding layer 702 and leaves the substrate from the lower surface thereof as shown. When such known micro-lenses are used in a liquid crystal display apparatus designed to contain a large number of small pixels in order to display finely defined images, the rays of light entering each micro-lens have to be converged to produce a very small spot of light. Then, obviously, the above pointed out problem adversely affects the effort of improving the brightness and the contrast of the image to be displayed on the liquid crystal display apparatus.
There arises another problem as will be described below when known micro-lenses are used for liquid crystal display apparatus and photosensors. Normally, the circuit substrate of a display apparatus or a photosensor is prepared by way of a semiconductor process independently from the process of producing micro-lenses before they are bonded together in a well aligned fashion to become integral with each other. A high degree of accuracy is required for aligning them with each other particularly when small pixels are used. For instance, if each pixel is about 10 microns large in an TN type transmission liquid crystal display apparatus, each pixel electrode is required to have a size of about 9 microns in order to satisfactorily separate adjacently located pixels. A phenomenon referred to as disclination of liquid crystal occurs in a peripheral area of the pixel electrodes to make it impossible to uniformly control the orientation of the liquid crystal and therefore to effectively utilize them in a peripheral area of about 2.5 microns. The contrast and the gradation of the image displayed on the display apparatus will be degraded if the light-shielding effect is not perfect in this area. Taking additionally the above pointed out problem of the diameter of each spot of light into consideration, the allowable dimensional error for the alignment of the micro-lenses and the circuit substrate will be less than 1 micron when the spot diameter is 3 micron because 9-2.5-2.5-3=1, which may be too rigorous to the process of manufacturing liquid crystal display apparatus. While there is a known technique of forming alignment marks, utilizing the light-shielding layer, the marks formed in the light-shielding layer are used for both the operation of patterning for the micro-lenses and that of aligning the circuit substrate and the micro-lenses because the micro-lenses are subjected to the patterning operation after forming the light-shielding layer. FIG. 14 shows alignment marks 801 in cross section. However, with the use of such alignment marks, it will be difficult to reduce the error of the alignment of the pixel electrodes and the micro-lenses to a desired level if the alignment marks are displaced relative to the micro-lenses even when the error of the alignment of the circuit substrate and the micro-lenses is reduced to less than 1 micron.
Thus, the above discussed problems have been baffling the past efforts for arranging more pixels in a liquid crystal display apparatus and improving the brightness and the contrast of the displayed image when known micro-lenses are used for a liquid crystal display apparatus.